1. Field of the Invention
The present invention generally relates to a power supply apparatus for driving a DC motor. More specifically, the invention is directed to a power supply apparatus for driving, for instance, a motor of a cassette player built in an automobile, in which a power loss occurred in a series regulator is minimized and malfunction caused by such a heat loss of circuit components is prevented.
2. Description of the Related Art
Various types of power supply apparatus for driving the DC motor have been proposed in this technical field, for instance, Japanese KOKAI (Disclosure) patent application No. 61-3349 (1986) and U.S. Pat. No. 4,591,768 to Kudelski.
FIG. 1 is a block diagram of the conventional power supply apparatus for driving the DC motor. In FIG. 1, a DC (direct current) voltage V.sub.IN from a battery (not shown in detail) for a car power source is applied via a DC voltage input terminal 1 to a switching regulator 2. An output voltage V.sub.out1 of this switching regulator 2 is applied to the subsequent series regulator 3. An output voltage V.sub.out2 of the series regulator 3 is further applied to a motor drive circuit 4. This motor drive circuit 4 is used for, for instance, driving a motor 5 of a cassette player built in a vehicle. Adjacent to the motor 5, there is employed a sensing element 6 for sensing a rotation speed of the motor 5. A sensor output from the sensor element 6 is supplied to a motor speed judging circuit 7. In the motor speed judging circuit 7, a comparison is performed between the sensor output from the sensor element 6 and a predetermined reference signal. Depending upon the comparison result, a predetermined output signal is supplied from this motor speed judging circuit 7 to a control voltage generating circuit 8. A control voltage V.sub.REG derived from the control voltage generating circuit 8 is applied to the above-described series regulator 3, whereby the output voltage V.sub.OUT2 from the series regulator 3 is properly controlled based upon this control voltage V.sub.REG.
FIG. 2 is a schematic circuit diagram of a typical switching regulator. In FIG. 2, an emitter of a switching transistor Q.sub.1 is connected to an input section (IN) of a power supply (not shown), and a diode D is connected between a collector of the switching transistor Q.sub.1 and a ground. A base of this switching transistor Q.sub.1 is connected via a resistor R.sub.1 to a collector of a controlling transistor Q.sub.2. A controlling signal is supplied via a resistor R.sub.2 to a base of this controlling transistor Q.sub.2 whereas an emitter thereof is grounded. Between a junction between the switching transistor Q.sub.1 and diode D, and a ground line, a series circuit consisting of a filtering inductor "L" and a capacitor "C" is parallel-connected. Another junction between the inductor "L" and capacitor "C" is connected to an output section (OUT) of the above-described power supply.
FIG. 3 is a schematic circuit diagram of a typical series regulator. In FIG. 3, a collector of a transistor Q.sub.3 is connected to the input section (IN) of the power supply whereas an emitter thereof is connected to the output section (OUT) of the power supply. A series circuit consisting of resistors R.sub.4 and R.sub.5 is connected between the output section (OUT) of the power supply and a ground line. A junction between these resistors R.sub.4 and R.sub.5 is connected to one terminal of a differential amplifier (DA) whereas a predetermined reference voltage "E.sub.REF " is applied to the other input terminal thereof. This voltage "E.sub.REF " corresponds to the above control voltage V.sub.REF. An output terminal of this differential amplifier DA is connected via a resistor R.sub.3 to a base of the transistor Q.sub.3.
Then, operations of the conventional power supply apparatus for driving the motor will now be described with reference to FIG. 1. It is assumed that the motor 5 is being rotated while input power is supplied from this power supply apparatus. The rotation speed of this motor 5 is detected by the detector element 6, and the resultant signal of this detector element 6 is supplied to the motor speed judging circuit 7. In the motor speed judging circuit 7, a comparison is carried out between this resultant signal from the detector element 6 and the predetermined reference signal, and a predetermined output signal is supplied therefrom to the subsequent control voltage generating circuit 8 in accordance with the comparison result. As previously described, the control voltage V.sub.REF derived from this control voltage generating circuit 8 is added to the series regulator 3, whereby the output voltage V.sub.OUT2 of the series regulator 3 is properly varied in response to this control voltage. Typically, this series regulator 3 shown in FIG. 3 employs the transistor Q.sub.3 connected between the power source input unit (IN) and power source output unit (OUT), and this transistor Q.sub. 3 will cause a voltage drop in the emitter-to-collector path. As a consequence, an unnecessary power loss may be induced therein, and thus this transistor Q.sub.3 is heated by herself, resulting in the malfunction of the circuitry.
As previously stated, the above-described conventional motor driving power supply apparatus may be applicable to the cassette player built in the vehicle. In such a car cassette player, generally speaking, the voltage to be applied to the motor 5 is low during the playback (reproducing) operation, i.e., the low speed mode, whereas this motor voltage becomes high during the fast forward (FF) operation, i.e., the high speed mode. Accordingly, in the prior art series regulator, the high voltage is continuously applied which is required during the above-described high speed mode. Then, during the low speed mode, the required voltage drop in this collector-to-emitter path of the transistor Q.sub.3 must be set higher. As a result, the unwanted power loss will be increased, and thus, the occurrence of the malfunction of the circuit elements due to the higher heat problem will be emphasized.
In the conventional power supply apparatus for driving the DC motor with the above-described circuit arrangement, since the voltage control to the motor is effected in the series regulator stage, the above-explained unnecessary power consumption caused by the rotation of the motor is induced, and the circuit elements of the series regulator are heated due to such an unnecessary power consumption, resulting in the malfunction of the power supply apparatus.
The present invention has been made in an attempt to solve the above-described conventional problems, and has an object to provide a novel power supply apparatus for driving a DC motor, wherein the voltage control to the motor is carried out at the switching regulator stage.
Another object of the present invention is to provide a power supply apparatus for driving a DC motor, wherein unnecessary power consumption is minimized to avoid heat problems of circuit components.